In respiratory care, it is often desirable to analyze and monitor the gas composition of a patient's exhaled and/or inhaled breathing gases. Various requirements for gas analyses exist in health care. For instance, measurement of respiratory CO2, O2, N2O and anesthetic agents, such as halothane, isoflurane, enflurane, sevoflurane or desflurane, is useful in the care of critically ill patients undergoing anesthesia or mechanical ventilation, while for emergency care such as manual ventilation it is typically sufficient to monitor breathing of a patient with a simple CO2 analysis.
Respiratory gases can be analyzed in accordance with many different measuring principles. The most common method of respiratory gas analysis, however, is through the medium of non-dispersive spectroscopy. This measuring principle is based on the fact that many gases absorb infrared energy at a wavelength specific to the substance concerned. Main flow gas analyzers based on non-dispersive spectroscopy measure light absorption at specific wavelengths directly in the patient's respiratory circuit. Capnography is the monitoring of the concentration or partial pressure of CO2 in respiratory gases, and provides real-time information regarding CO2 exhalation and respiratory rates as well as a rapid and reliable assessment of a patient's ventilatory, circulatory and metabolic function. Although the terms capnography and capnometry are sometimes considered synonymous, capnometry suggests measurement without a continuous written record or waveform. Typically in capnography and capnometry, a main flow measuring head is placed as close as possible to the patient's mouth or trachea to sample exhaled and/or inhaled breathing gases and calculate gas concentrations directly in the respiratory circuit of the patient.
Measurement of end tidal CO2 can also provide useful information such as regarding CO2 production, pulmonary (lung) perfusion, alveolar ventilation, respiratory patterns, and elimination of CO2 from an anesthesia breathing circuit or ventilator. The gas sample measured at the end of a person's exhalation is called the “end-tidal” gas sample. The amount of carbon dioxide in a person's breath can indicate the overall efficiency of the cardio-pulmonary system and quality of breathing. For example, the concentration of carbon dioxide can indicate shallow breathing and poor oxygen intake. Thus, capnographers are used in hospitals and other medical institutions for monitoring the condition of a patient's respiratory system, pulmonary perfusion, and metabolism, and are most often used for patients in intensive care and under anesthesia.
In many clinical and emergency settings, respiratory assistance is accomplished through use of bag-valve mask (BVM) ventilation systems. Main flow measuring heads can be useful for implementation in BVM ventilation systems and other manual ventilation systems to measure end tidal respiratory gases during respiratory assistance. BVM ventilation is a life-saving skill of an emergency physician or pre-hospital care provider that can easily be overlooked because of its apparent simplicity. However, BVM ventilation is a difficult skill to master, and poor BVM ventilation technique can lead to the need for more invasive means of airway management and their inherent complications. Implementing BVM ventilation with a low rate of bag compression can lead to hypoventilation and inadequate oxygen supply to the patient. Hyperventilation due to overzealous BVM ventilation can be harmful by increasing intra-thoracic pressure, which decreases venous blood to the heart and subsequently decreases cerebral and coronary perfusion pressures. The appropriate rate of bag compression for proper patient oxygenation differs based on factors such as the age of the patient and the size of the bag. Therefore, there is a need for measuring heads that are capable of providing instructions and feedback to manual ventilation providers.